Devices known as pigs are commonly passed through pipelines in order to service or inspect the pipeline. A pig may contain the necessary servicing or inspection equipment or may draw other devices behind it. In pipelines for conveying fluids, the pig is commonly driven along the pipeline by the flow of the pipeline fluid. To achieve this, the pig may be fitted with one or more flexible disks or cups attached to the body of the pig and extending outwardly to the interior pipe wall so as to form a sufficient seal in the pipeline. Such arrangements have been known and used for many years. If the seal is substantially complete, the pig will travel at the speed of the pipeline fluid.
If a pig becomes stuck in a pipeline, it can restrict or impede the flow through the pipeline. Often, contingency plans are implemented in order to recover the pig. Contingency plans may involve shutting down the pipeline, depressurizing the pipeline, flushing the pipeline, lifting the pipeline, and cutting the pipeline at the point where the pig is stuck in order to remove it. Once the pig is removed, the pipeline is then reconnected and put back in service. This contingency plan is costly due to inhibited pipeline flow and downtime while the pipeline is cleared. A by-pass pig with holes that allow some flow through can be used in place of a traditional pipeline pig. However, the holes in a by-pass pig reduce the maximum fluid force that can be applied to the pig to drive it through the pipeline. As a result, the chances of a by-pass pig becoming stuck in a pipeline are greater than that of a traditional pipeline pig.